Dams, weirs, and culverts that block fish migration routes cause stress, physical injury, and population-level welfare impacts by preventing fish from completing their natural life cycles.
Migratory fish such as Atlantic salmon, sea trout, and eels have evolved to travel hundreds of kilometers between feeding and spawning grounds. Barriers to this migration cause multiple welfare harms: stress and exhaustion from failed passage attempts, physical injury at turbines and weirs, and ultimately reproductive failure that leaves fish unable to complete their biological imperative. Fish held below impassable barriers show chronic stress indicators. Fish passage infrastructure like fish ladders offers partial solutions, but their effectiveness varies widely. The most welfare-positive interventions are those that remove barriers entirely or ensure low-impact passage for the entire migrating population.